Green’s Dictionary of Slang

1. an all-purpose intensifier, e.g. seriously rich.

1925		N. Lucas Autobiog. of a Thief 224: Do you know, Lucas, the Yard has been seriously after you for several months!
1982		Barr & York Sloane Ranger Hbk 159: serious, seriously adj & adv. The Sloane way of exaggeration. ‘He’s seriously rich.’.
2005		H. Mantel Beyond Black 69: Recarro seats. Full spec. Seriously speedy.

2. an expression of agreement.

1996

Eble Campus Sl. Mar. 7: seriously – expression of agreement: X:‘This weather is horrible.’ Y: ‘Seriously.’.